group.rb

class Group
  include DataMapper::Resource
  # ... no need to paste everything, right?

  has n,     :users,  :through => Resource
end

groups.rb

  def leave(grouplink) # grouplink is a permalink
    @group = Group.first(:grouplink => grouplink)
    raise NotFound unless @group
    @group.users.delete session.user
    redirect resource(@group), :message => (@group.save) ?
        {:notice => "Sorry to see you go"} :
        {:warning => "Looks like you're stuck here"}
  end

groups_spec.rb

context "when logged in as group member", :given => "a group user is logged in" do
  describe "resource(@group, :leave)" do
    before(:each) do
      @response = request(resource(Group.first, :leave), :method => "PUT")
    end
    
    it "should redirect to the group page" do
      @response.should redirect_to(resource(Group.first), :message => {:notice => "Sorry to see you go"})
    end
  end
end